(1) Field of the Invention
The present invention relates to an apparatus for processing a video signal. The video signal processing apparatus can be used in a color television receiver capable of displaying images on the basis of video signals separated into a luminance signal and a chromatic signal and which can be adapted to a function of superimposing computer images on the images of the video signals.
(2) Background of the Art
Color television receivers capable of displaying images of video signals derived from an external computer superimposed on those derived from a VTR (Video Tape Recorder) have been proposed.
For the transmission and reception of the computer images, an RGB (Red, Green, and Blue) signal is normally used.
As a representative interface, RGB multi-pin connectors, 21 pins, are extensively adopted in character broadcastings, videotex, and personal computers.
The RGB multi-pin connector installed on the television receiver includes a first terminal through which a video signal derived from the VTR (including a synchronization signal) is outputted to the computer in order to synchronize between those video signals when the computer images are superimposed on the VTR images (RGB multi-pin connector video output terminal), a second terminal through which the synthesized video signal to be synthesized so as to be superimposed on the VTR images is inputted to the television receiver (RGB multi-pin connector video input terminal), and a third terminal (RGB input terminal) through which the computer images are inputted to the television receiver. It is noted that the video signals inputted and outputted through the RGB multi-pin connector are composite picture signals.
A previously proposed switching circuit for the video signal includes a first switch for selecting either of a television signal or a composite video signal from the video equipment such a the VTR and a second switch for selecting either of the selected input signal via the first switch or the video signal inputted via the computer to be superimposed on the television signal or VTR video signal.
The selected input of the first switch is supplied to the computer as an output video signal (RGB multi-pin connector video output signal). After the supplied input is used for the synchronization processing of the computer image with the VTR or television signal, the synchronized computer video signal is inputted to the second switch as an input video signal (RGB multi-pin connector video input signal) to be superimposed on the VTR or television signal. The first switch and second switch are changed over in response to a command from an operation switch installed on a front or rear surface of the television receiver.
That is to say, a portion of a movable contact of the first switch is placed at an input contact receiving the television signal when no video switching signal is present. When the video switching signal is present, the position of the movable contact of the first switch is placed at the other input contact receiving the composite video signal. On the other hand, a movable contact position of the second switch is placed at the first switch contact side when no RGB switching signal is present. When the RGB switching signal is present, the movable contact position of the second switch is placed at the RGB multi-pin connector video input contact side.
The previously proposed television receiver carries out the image display with the composite video signal separated into its luminance signal and its chromatic signal by means of a comb filter. Recently, however, a new television receiver has been proposed having a so called S terminal input through which a video signal derived from the VTR is already separated into the luminance signal and the chromatic signal to make a further improvement of a picture quality of the television receiver.
However, the following problems arise in the switching circuit of the video signal for the latter proposed television receiver.
(1) Since it is necessary to include the synchronization signal in the video output signal outputted via the RGB multi-pin connector video output terminal to superimpose the video signal derived from an external computer on the S terminal input 4, the RGB multi-pin connector video signal is only the luminance signal derived from the S input terminal. Therefore, the superimposed image of the VTR or television is merely a monochrome image.
(2) To solve the problem of (1), a circuit for synthesizing the luminance signal and the chromatic signal which are separated in the television receiver to produce a video composite output signal for the RGB multi-pin connector used to superimposed onto the images derived from the VTR and so on. However, such a circuit as described above is complex in circuit construction and is therefore expensive.
(3) To solve the above-described problem of (1), the composite video signal which has normally been prepared in the VTR together with the separated video signal for the S terminal is utilized to cause an image superimposition. At this time, means for rewiring the connections and switching the connections have been considered. However, the operation for the rewiring and switching means becomes troublesome.